Last month I posted an article about a mysterious "Free Public Wifi" signal that I had found while travelling under central London on the (very deep) Piccadilly Line. It seems that other travellers have been noticing the same signal and there's been some conjecture as to whether this could be some sort of trial for Boris Johnson's promised capital-wide network for the Olympics.
Today I got a message from reader Martha Hampson who had noticed the same signal on the Northern Line, also deep under ground, and had searched the internet for references. She has telephoned Transport for London and they say it's nothing to do with them. They suggested that wifi signals do sometimes leak through ventilation shafts. This sounds a bit unlikely because of the depth of the lines and, if this were the case, the signal would be fleeting as the train passed the shaft.
However, she has found a possible alternative explanation which sounds pretty fantastic but could just be true. This blog post by Dwight Silverman of TechBlog in 2006 produces an answer. The "Free Public WiFi" signal, which always leads nowhere, is actually being transmitted by Windows computers in the vicinity. I won't cover the technicalities (and I'm not sure I understand them anyway), so read the full story here.
I'm reminded by this that I've seen "Free Public Wifi" access points in other parts of London and, even in foreign countries where, in retrospect, I would have expected the name to be in the local language. But it is always in English and identical. Dwight Silverman reckons this is not a virus, but it is viral in the sense that it propagates itself through Windows computers which latch on to the name and then retransmit it. I wouldn't have thought this possible, but the argument is compelling.
So the explanation of this munificent free wifi on the London Tube could be quite prosaic: a fellow traveller using a Windows laptop somewhere else in the same carriage. So next time you see the signal, have a scout round to see if there is a computer nearby. In the meantime, I will keep my fingers crossed that Macs are immune.